1. Field of the Invention
The present invention relates to partially implantable hearing devices generally and particularly to such devices which use electromagnetic drivers and target permanent magnets to stimulate the ossicular chain of the middle ear.
2. Description of the Prior Art
Conventional prior art hearing aid is composed of a microphone, an amplifier, a battery as a power source, and a speaker or earphone (commonly referred to as a receiver in the hearing aid industry). The implantable hearing device has the same basic components, except that the speaker is replaced by a driving vibrating component, such as an electromagnetic coil or a piezoelectric system of bimorph design. Environmental sound energy, as it passes through either device, is converted by the microphone into an electrical signal which is routed to an amplifier. In the conventional hearing aid, the speaker transduces the amplified electrical signals into acoustic energy, which is then transmitted to the tympanic membrane and ossicular chain. In the implantable middle ear hearing device, the speaker is eliminated, being replaced by the vibratory component which drives the ossicular chain.
In 1987, Hough et al. reported on a middle ear implantable hearing device using electromagnetic principles applied to humans undergoing middle ear surgery under local anesthesia. Although the device was functional, its electrical power consumption was excessive.
Ko, Maniglia and Zhang also reported, in 1987, their experience with an electromagnetic middle ear hearing devices using direct stimulation of the stapes. Furthermore, Maniglia et. al. have done extensive experiments in this field involving animals such as cats and rabbits as well as preserved and fresh human temporal bones. Goode, et al. have experimented with a piezoelectric system to produce stapes vibration in fresh human temporal bones. Heide, et al. in 1988 presented the advantages of an electromagnetic hearing aid in the ear canal driving a magnet glued to the ear drum. Finally, Goode has recently reported encouraging results with another design in which another electromagnetic canal device similar in principle and design that stimulates a samarium cobalt magnet attached to a silicone mold which is fitted on the ear drum with mineral oil. This is known as an EAR LENS system of attachment or gluing to the ear drum. However, the magnet glued to the ear drum only stays in place temporarily, and a better system of adhering is necessary.
Totally concealed, partially implantable middle ear hearing device are also known. These devices are described in U.S. Pat. Nos. 4,957,478 and 5,015,224 issued to Anthony J. Maniglia and have a replaceable outer ear canal unit and an implanted magnet attached to the ossicles of the middle ear specifically the handle of the malleus. The replaceable ear canal unit receives acoustic energy or sound waves that enter the ear and travel down the outer ear canal to the unit. A microphone detects the sound waves and, with the help of a battery and an electronic amplifier, transforms the sound waves into amplified electrical signals. The electrical signals activate an electromagnetic driving coil, i.e., a coil of wire wrapped around a ferrite alloy core, which creates a magnetic field that varies in response to the sound waves detected by the microphone. The magnetic field created by the electromagnetic coil interacts with the magnetic field created by the magnet, generating a force which vibrates the target magnet and the malleus bone to which it is attached. To insure that the target magnet is securely attached to the malleus bone, the titanium case has a self tapping mini-screw to be inserted into a man-made micro cavity created in the malleus bone. Once the screw is inserted into the malleus bone, it is allowed to osseo-integrate for a three month period. After this period of time, the replaceable outer ear canal unit is put into use. The magnet would weight about 30 to 35 mg and the distance between this magnet and the external ear canal unit would be 3 to 5 mm.
Another variation of this system consists of placing the magnet encased in a titanium dish anchored to the lateral aspect of the incus. Two holes are made in the body of the incus. The titanium extension of the dish is secured to the incus by two self tapping screws introduced through the previously made holes.
In another variation of this system a totally concealed, partially implantable middle ear hearing device having a replaceable outer ear canal unit has means for generating radio frequency waves responsive to acoustic energy or sound waves that enter the ear and travel down the outer ear canal to the unit. Again, a microphone detects the sound waves and, with the help of a battery and an electronic amplifier, transforms the sound waves into amplified electrical signals. In this aspect, however, the amplified electrical signals are sent to an external induction coil or radio signal transmitting antenna to be converted into amplitude modulation (AM) radio frequency waves that are transmitted to an internal induction coil implanted under the skin in the outer ear canal wall. An implanted electromagnetic driving coil, connected to the internal induction coil, again creates a magnetic field that varies in response to the sound waves detected by the microphone. This magnetic field interacts with another magnetic field created by a magnet attached to a bone in the ossicular chain in the middle ear. This interaction causes a force which vibrates the magnet and the bone to which is it attached. In one case, the bone to which the magnet is attached is the stapes; in another case the magnet is attached to the incus; while in a third case there is an electromagnetic-mechanical system having a very thin metal diaphragm attached to a titanium coil spring secured to the incus body, using a self-tapping titanium screw introduced through a hole, KTP 532 laser made. In another design the titanium spring coil is attached to a cup-bumper which "sits" on the stapes head.
Still another variation of this device has a partially concealed, partially implantable hearing device having a replaceable hidden external unit adapted to be located externally and medially to an upper portion of a pinna of an ear, rather than being located inside the outer ear canal of the ear. A microphone detects the sound waves and, with the help of a battery, an electronic amplifier and an external induction coil, the sound waves are converted into amplitude modulation (AM) radio frequency type waves for transmission to an internal induction coil implanted under the skin behind the ear. An implanted electromagnetic driving coil, connected thereto, creates a magnetic field that varies in response to the sound waves detected by the microphone. The magnetic field created by a magnet attached to a bone in the ossicular chain in the ear interacts with the magnetic field created by the electromagnetic driving coil, causing the magnet and the bone to which it is attached to vibrate. Again, in one case, the bone is the stapes bone; in another case the bone is the incus; while in a third case the electromagnetic-mechanical system mentioned above is employed.
Several problems occur with the above described designs. First, the implantation of the magnet on the head of the stapes require disarticulation of the ossicular chain. The technique is very invasive and rather destructive, creating a conductive hearing loss. If the device were to malfunction for any reason, a maximum air-bone gap would occur, because the conductive hearing loss component created by the disarticulation of the ossicular chain would be further aggravated by the load of the magnet on the head of the stapes. The clinical application for this proposed device is thus limited to patients with a conductive overlay or mixed hearing loss with erosion of the incus and ossicular discontinuity or absence of the incus and malleus. Thus, a device was needed that would be applicable to a much larger number of patients suffering from sensorineural hearing loss, which would avoid interruption of the ossicular chain and minimize invasive techniques in affixing the magnet on the ossicles.
To accomplish this goal an efficient, biocompatible, adhesive type of cement was needed to affix the magnet assembly on the body of the incus. Also, the existing magnets were found to have coercivity factors which necessitated a large size to effect the needed power to provide the necessary hearing boost. This not only made the adhesion difficult but added mass to the ossicular chain. Thus a small, lightweight high coercivity magnet was needed to make the electromagnetic device work efficiently.